1. Field of the Invention
The present invention relates to a method of removing sulfur oxides and particulates from gases containing the same. It particularly relates to a method wherein a hot gas containing sulfur oxides and particulates is controllably reacted in a first zone and then passed sequentially into a second zone, the sulfur oxides in the gas being reacted in each zone with a selected chemical adsorbent therefor.
2. Prior Art
Sulfur oxides, principally as sulfur dioxide, are present in the waste gases discharged from many metal refining and chemical plants and in the flue gases from power plants generating electricity by the combustion of fossil fuels. In addition, hot sulfur-containing gases may be formed in the partial combustion or gasification of sulfur-containing fuels, such as coal. The control of air pollution resulting from the discharge of sulfur oxides into the atmosphere has become increasingly urgent. An additional incentive for the removal of sulfur oxides from waste gases is the recovery of sulfur values otherwise lost by discharge to the atmosphere. However, particularly with respect to the flue gases from power plants, which based on the combustion of an average coal may contain as much as 3000 p.p.m. sulfur dioxide and 30 p.p.m. sulfur trioxide by volume, the large volumes of these flue gases relative to the quantity of sulfur which they contain make removal or recovery of the sulfur compounds expensive. Also, the total quantity of possible by-products, such as elemental sulfur and sulfuric acid, that could ultimately be obtained from the recoverable sulfur values would exceed the demand for such by-products.
Many processes have been proposed and investigated over a period of many years for the desulfurization of flue gases. Several solid-gas contact processes have been proposed in which the sulfur dioxide present in the flue gas is removed either by chemical reaction with a solid adsorbent or by adsorption on an active surface followed by oxidation of the adsorbed sulfur dioxide. In one such process, shown in U.S. Pat. No. 2,718,453, finely powdered calcium carbonate is blown into the combustion gas to form calcium sulfate or calcium sulfite.
Another example of a solid-gas contact process is shown in U.S. Pat. No. 3,310,365, which is directed to eliminating sulfur trioxide-induced corrosion. In this process a gas stream containing about 20 p.p.m. sulfur trioxide is cooled below the acid dew point value of the gas to form a hydrated sulfur trioxide aerosol, and a finely divided dolomitic limestone or other alkaline additive is injected into the gas stream, using about two and one-half to three times the stoichiometric amount required for complete neutralization. As further noted in this patent: "The particulate matter in the flue gas, including the injected alkaline additive, is separated from the gas by impingement upon the fabric filter surface of the bags, the alkaline additive functioning as a filter aid and building up a matrix through which the sulfur trioxide-laden gas must pass, bringing about the desired neutralization reaction for removal of the sulfur trioxide."
U.S. Pat. No. 3,852,410 describes another gas-solid contact process for continuously removing sulfur dioxide and particulate contaminants from industrial stack gases containing the same by use of a soluble alkaline sodium compound as a sulfur dioxide reactant, which is subsequently regenerated. Fabric filter dust-collecting surfaces are preloaded with the finely divided soluble alkaline sodium compound, and a waste gas containing sulfur dioxide, carbon dioxide and particulate contaminants is passed through the dust collector. The particulate contaminants are restrained by the dust collector, and a portion of the sulfur dioxide reacts with the soluble alkaline sodium compound.
In general, a reaction between a solid and a gas is relatively slow and inefficient, being limited by the available surface area of the solid. Also certain of the resultant products do not readily lend themselves to regeneration of the starting material or recovery of the removed sulfur values.
In the molten carbonate process shown in U.S. Pat. Nos. 3,438,722, 3,428,727, and 3,438,728, sulfur oxide impurities are removed from a hot combustion gas by contacting the gas at a temperature of at least 350.degree. C. with a molten salt mixture containing alkali metal carbonates as the active adsorbent. The spent adsorbent is then regenerated chemically and recirculated. The adaptation of such a process to many existing power-plant utility installations often presents certain economic disadvantages because of the requirements for modifying the boiling systems of these utility plants in order to obtain the flue gas to be treated at the required elevated temperature for the molten salt absorption, rather than at its generally much lower exit temperature from the boiler.
Wet absorption processes have been used for treating lower temperature flue gases. In a typical wet absorption process, the flue gas is washed with an aqueous alkaline solution or slurry. Aqueous slurries of calcium oxide, calcium hydroxide or calcium carbonate have been used for removal of sulfur dioxide from flue gas in several power plants. Also, aqueous sodium sulfite or ammonia solutions have been utilized as washing fluids.
In the wet absorption process shown in U.S. Pat. No. 3,533,748, a waste gas stream containing sulfur oxides is scrubbed with an aqueous solution of a soluble alkali, such as sodium carbonate or sodium hydroxide, to form sulfite and sulfate in solution. The resulting solution is then evaporated to precipitate solid alkali metal sulfite and sulfate salts, which are separated from the solution and further processed.
While these wet absorption processes have some advantages, they suffer from a common drawback of producing a liquid effluent containing a large amount of water relative to the sulfur oxide absorbed, which effluent is not amenable to simple high-temperature reduction and regeneration. Thus, difficulties arise where economic and efficient recovery of the dissolved absorbent and sulfur values from an aqueous solution is attempted. In many such processes, the recovery of elemental sulfur, a preferred product, is not economical. Further, the wet processes frequently produce a water-saturated product gas which must be heated prior to discharge to the atmosphere to avoid an objectionable plume.
In U.S. Pat. No. 3,305,307, there is shown a process for the manufacture of solid alkali metal sulfite with negligible formation of alkali metal sulfate. A finely dispersed concentrated aqueous solution of an alkali metal compound such as sodium or potassium carbonate, hydroxide, or bicarbonate is passed into a substantially dry gas containing an equivalent or greater amount of sulfur dioxide, the dry gas being maintained at a temperature such that solid alkali metal sulfite is formed. To obtain a pure alkali metal sulfite by such a process, an excess reactant amount of SO.sub.2 compared with the alkali metal compound is required. Also, to avoid the formation of alkali metal sulfate, the gas containing the SO.sub.2 reactant must be relatively free of sulfur trioxide and oxidation-promoting substances such as nitrogen oxides and metal oxides, the latter being found in fly ash. In addition, a relatively low temperature of reaction is generally required, higher temperatures promoting formation of sulfate. This patent, which is directed to the manufacture of a pure chemical compound, is not concerned with the problems associated with treating waste gas streams, such as the low concentrations of sulfur oxides to be removed as well as the presence in the gas stream of significant amounts of inert particulate matter.
U.S. Pat. No. 3,932,587 is directed to a closed-cycle process for removing, in a single spray-drying step, a sulfur oxide impurity from a hot waste gas. The resultant products are removed from the flue gas in a subsequent step using a conventional gas-solid separator.
U.S. Pat. No. 3,880,629 is directed to an air pollution control process for the treatment of a high-temperature glass furnace gas. A sodium alkali is used as an SO.sub.x absorbent, wet or dry, and is injected into the hot flue gas issuing from the glass furnace. This flue gas contains fine particulates of glass components and SO.sub.x, which is evolved from the Na.sub.2 SO.sub.4 fining agent used in the glass batch. The absorbent reaction product and the glass particulates are thereafter collected in a baghouse as a dry filter cake. After appropriate sizing, this filter cake is recycled to the glass melt. The preferred sodium alkali absorbent is nahcolite ore, which is principally sodium bicarbonate. When the nahcolite ore is used as a dry absorbent for a gas-solid phase reaction, it is continuously fed as a fine powder into the flue gas stream. The gas temperature is maintained at about 260.degree. C. (500.degree. F.). The gas containing the absorbent and reaction products is then directed onto bags, which have been precoated with a thin layer of nahcolite ore. The baghouse, with the nahcolite ore layer on the bags, serves the dual function of acting as a filter aid for collecting the glass batch particulates and also for removing the SO.sub.2. Where the sodium alkali absorbent is used in the wet state, it is sprayed as a liquid alkali solution into the hot flue gas, spray heads being used to break up the aqueous sodium alkali solution into fine droplets so as to obtain effective contact with the hot flue gas. The formed sodium sulfite and sulfate is dried by the heat of the flue gas and is then collected along with the glass batch fines in the baghouse, the baghouse in this embodiment acting as a collector rather than as a reactor for SO.sub.x emission control. The baghouse filter cake contains the sodium sulfate reaction product, residual unreated sodium carbonate, and glass batch fines, and this filter cake may then be recycled to the glass melt. The use of the wet spray-dry technique followed by collection of the reaction products in a baghouse is also indicated as applicable to the treatment of powerplant flue gas obtained from the burning of oil and coal fuels and containing SO.sub.x and fly ash emissions.
Other gas purification patents of interest, but not considered any more pertinent than those discussed, include U.S. Pat. Nos. 931,515, 984,498, 2,875,844, 2,875,847, 2,919,174, 3,933,978, 3,969,482, and 3,976,747. Other patents and sulfur oxide removal processes are discussed in considerable detail in the patents reviewed, both with respect to specialized requirements for treating the gases evolved from particular industrial processes as well as the requirements for the removal of sulfur oxide contaminants from flue gases emitted from oil- and coal-burning power plants. However, despite all of this activity in gas purification over many years, and the many plans and processes proposed, both speculative as well as experimentally evaluated, the need still exists for an effective, commercially feasible method for controlling both particulate and sulfur oxide emissions from power-plant flue gases in a manner that is efficient, simple and inexpensive, and yet is readily adaptable to the flue gas characteristics of existing power-plant installations on a retrofit basis. There is also a present need for a throw-away once-through process for sulfur dioxide removal because of the substantial additional capital investment required for recycle and absorbent recovery. In addition, such an air pollution control process must be versatile with respect to being able to meet stringent Governmental environmental requirements while at the same time being able to use a wide variety of absorbents, essentially interchangeable for the specific requirements of a given power plant, but yet without requiring substantial modifications in the process. To date no sulfur dioxide removal process is commercially available that has achieved this required versatility while meeting the desired economic and environmental restraints.